1. Field of the Invention
The present invention relates to the field of air cargo screening systems for the detection of the presence of explosive devices therein.
2. Prior Art
In recent years, there has been an increasing threat and occurrence of actual bombings by various terrorist groups. Of particular concern is the possible placement of bombs on commercial aircraft, both because the actual bombing on the aircraft normally leads to a great loss of life and to great property damage, and because the mere threat or potential of such bombings has at least at times had gross adverse affects on the tourist and the airline industries.
One type of bomb which terrorists have used with success is a bomb which has a pressure sensitive trigger device which may require a pressure cycling between normal atmospheric (airport level) pressure and a reduced pressure characteristic of flight at normal altitudes a predetermined number of times, such as one or two, before the bomb is initiated upon an additional drop in pressure indicative of another flight at altitude. It is believed that in at least some cases, the pressure sensitive trigger device does not itself set off the bomb, but rather initiates a timing device set to trigger the bomb before the then current flight is over. Thus, a bomb may be placed in air cargo for a particular destination, to go off in the second or third leg of the trip to the air cargo destination, with the bomb being successfully set off during the desired leg of the trip, independent of intervening flight delays and the like.
Inspection techniques for the inspection of passengers, carry on baggage, stowed (checked) baggage and other more general forms of air cargo are well known. Such techniques generally include x-raying of baggage, the use of metal detectors for searching passengers themselves, and the use of dogs for sniffing out explosive materials. However, the shear volume and the nature of air cargo in general typically limits such inspection to specialized categories, such as baggage and carry on articles, as air cargo is voluminous and varies a great deal in packaging method, metal content, etc. Consequently, there has been no practical way to successfully inspect general air cargo for pressure sensitive bombs of the type described, as such bombs may be packaged with other materials making the discovery of the same almost impossible without the unpacking, even perhaps the disassembling of each item of any substantial size being shipped.